Machines for determining the proportions of particles in different size ranges in a particulate sample are known as particle size analyzers. U.S. Pat. No. 4,487,323, to Gevan R. Marrs, issued Dec. 11, 1984, and entitled "Automatic Particle-Size Analyzer", discloses apparatus for automatically determining the proportions by weight of particles which pass through the different screens of a series of graduated mesh sizes. That analyzer has a rotatable, polygonal screen drum which mounts a series of graduated screens on its sides. The drum is indexed about a horizontal axis so that, beginning with the screen with the smallest openings, screens of increasingly larger mesh size are successively moved to a generally horizontal, lowermost or "down" screening position. The same to be analyzed is introduced into the interior of the drum and the drum is vibrated longitudinally or otherwise shaken to impart an effective screening motion which separates the particles on the screen in the "down" position.
A collector beneath the active screen receives and weighs the particles which pass through that screen; from this weight the sample percentage which has passed through that screen can be calculated. The drum is then automatically indexed to present the next larger screen at the down position, the remaining sample falls onto that screen and is screened, the proportion which passes through it is similarly determined, and so on until the sample has been screened on all the screens. The last side of the drum may be open (without a screen) so that it dumps the remaining "overs" into the collector.
In the operation of apparatus of this type it is observed that some "blinding" or screen clogging often occurs. Particles which are just slightly larger than the screen openings may become lodged in those openings; unless dislodged, they are thereby effectively removed from the sample. The useful screen area is also dimished, which can affect the standardizing of the cycle. It is therefore desirable to provide means for cleaning each screen, and for doing so in a manner such that the accuracy of the analysis is not adversely effected.
In using this type of apparatus, the drum is usually automatically indexed through the entire sequence of screens, and the whole operation may be carried out without manual control. It is therefore important that the cleaning also be carried out automatically, and in a manner to clean each screen surface without interfering with the screening action or the indexing cycle.